Measurement of the exact contour of a deep mirror to determine the accuracy of the inner and outer surface slopes, failures, and roughness of rotational or partially rotational deep mirrors has, in the past, been a very tedious and difficult process. In utilizing prior measurement techniques for the inspection of optical surfaces such as mirrors and prisms, considerable difficulties are experienced in detecting defects in these surfaces. This is due, in part, to the fact that the defects have only a very small influence on the incident light. Although, in theory, it is possible to discriminate between light which follows the desired path of a perfectly inspected optical component from light which is slightly deviated as the result of imperfections in the surface of the optical device, this is virtually impossible as a practical matter.
Present methods for measuring optical surfaces utilize an external light beam that is either scanned across the optical surface or the optical surface is moved relative to the light beam and the reflected beam measured. Inherent imperfections in the various mechanisms that are utilized to either scan the light beam or move the optical surface can sometimes result in greater errors than those resulting from the imperfections on the actual optical surface. Examples of these measurement devices are disclosed in U.S. Pat. Nos. 3,892,494 and 3,877,788.
In view of the above disadvantages, there exists a need for a measurement system that alleviates or compensates for the inaccuracies in the measurement system itself such that a higher degree of accuracy and measurement efficiency can be achieved.